Any component which I suspect may not last the distance is replaced and put aside for local use.
I have done that to some extent, but have found a few problems with it at least for my personal situation. First, on tours longer than some length you can't plan on everything lasting the whole way. And second, I found that for me stuff that came off when partially worn out never got used again and things like tires with a couple thousand miles of wear left in them would wind up going to waste, hanging in the shop for years until dry rotted.
It probably doesn't help that I don't ride all that much at home and generally do long tours. So for me it seems to make more sense to just replace things as needed whether at home or on tour.
That said if something is really close to end of life I would probably replace it before a long tour. For example, I wouldn't hesitate to start a coast to coast tour with tires that will make it half way but would probably replace them if they looked like they had less than a thousand miles left in them or were getting an excessive number of flats.
I have fitted inline cable adjusters near the shifters so I can fine-tune shifting as I ride.
I have bosses on the frame that have adjusters on some of my bikes and probably should add inline adjusters on the ones that don't. I find the ones on the frame to be easier to adjust with one hand while riding, so I prefer those when available.